1. Field of the Invention
The present invention relates to a navigation system for use with a mobile object, and more particularly to a navigation system including a data processor for producing data to estimate the position of a reception point highly accurately.
2. Description of the Prior Art
There are known stand alone type positioning devices for navigating various mobile objects such as automobiles, airplanes, ships, or the like in a global geometrical region. A stand alone type navigation system on a mobile object has an direction sensor and a speed sensor. The stand alone type navigation system determines a two-dimensional displacement (vector quantity) of the mobile object based on direction data from the direction sensor and speed data from speed sensor, and finds the present position of the mobile object by adding the determined two-dimensional displacement to the positional data of a reference position. For example, the stand alone type positioning system on an automobile produces a predetermined number of pulses per revolution of the drive shaft of the automobile, and calculates the distance that the automobile has traveled by multiplying a distance calculated from the total number of pulses produced while the automobile has moved from the reference position to the present position, by a distance corrective coefficient. Since, however, the two-dimensional displacement contains accumulated errors of the direction and speed sensors, the stand alone type positioning system requires some means for eliminating the accumulated errors. More specifically, because of the accumulated errors of the sensors, if a distance corrective coefficient that has once been established for the speed sensor remained unchanged, then the calculated distance would include an error. Distances calculated for automobiles by stand alone type positioning systems are also subjected to errors depending on the number of passengers on board and the conditions of roads on which the automobiles run. Consequently, when an automobile with a stand alone type positioning system makes a turn at an intersection, it has heretofore been necessary for the user of the navigation system to manually correct the distance corrective coefficient if the user finds the navigation system either overrunning or underrunning.
Such accumulated errors cause no problem with global positioning systems (GPS) for determining the position of a mobile object in a global geometrical region based on radio waves transmitted from three or more GPS satellites. The global positioning system determines the position of a reception point where its radio-wave receiver has received the radio waves, based on quasi-distance data between the GPS satellites and the reception point, including a time offset of the receiver, and positional data of the GPS satellites
The global positioning systems produce direction data by measuring the rate of change of the quasi-distance data based on Doppler shifts observed in the radio waves from the GPS satellites. Consequently, when the reception point moves slowly, the direction data produced by the global positioning system is of low accuracy. Furthermore, when the reception point is at rest, the speed data obtained by the global positioning systems becomes larger than the actual speed due to the multipath effect of the radio waves, and the global positioning systems generate wrong direction data.